Support provision of audio source

[tommasobertoni]

The consumer of covox may provide an audio source for the recognition, instead of letting the engine listen directly to the microphone:

var covox = new CovoxEngine(new Configuration
{
    // ...
    AudioSource = Audio.FromDefaultMicrophone() // When AudioSource is null, this option is auto-selected
               // Audio.FromMicrophone(microphoneId)
               // Audio.FromStream(myCustomStream)
};